The present invention relates to printing inks. More specifically the present invention relates to printing inks wherein the principal solvent or vehicle is water, hereinafter denoted water-based and/or aqueous printing inks.
Printing inks are employed in the graphic arts as the medium by which desired colored representations are formed on a surface. The printing ink forms a colored coating in predetermined fashion on the selected substrate. Mechanical means carrying out a printing process applies the ink to the selected surface.
One way to classify inks is according to the type of printing process to be carried out. Certain types of printing processes require relatively viscous inks of a paste-like consistency while others demand inks of an oleophilic nature; still others entail the employment of relatively fluid, quick-drying inks. Letter press, gravure and flexographic printing processes, among others, usually necessitate fast drying inks.
Although inks can dry through one or more mechanisms, such as absorption, evaporation, oxidation, polymerization and the like, most often the evaporation mode is employed for the more fluid inks used in the letter press, flexographic or gravure processes. From a conventional viewpoint, this has led to the use of relatively volatile organic solvents as the ink solvent or vehicle. When the ink is applied to the substrate, the solvent, with the aid of heat where needed, evaporates to leave the dried coloration.
Although water-based printing inks are known, they have not been economically desirable due to the relatively high energy input needed for rapid evaporation. As noted above, volatile organic solvent ink vehicles requiring little or no added energy for quick evaporation at room temperature have been widely used. However, undesired characteristics and disadvantages in their use have led to the consideration of new systems for drying or curing inks. Pollution of the environment by evaporated organic solvents presents obvious health and safety problems. Governmental bodies are imposing strict standards of allowable dissipation into the air. Expensive and complex reclaiming equipment must often be considered to meet the stringent governmental regulations.
A number of approaches have been considered to avoid the pollution problem caused by the rapid drying organic solvents. One such approach is the use of solventless inks, dryable through cross-linking or ultraviolet radiation. Another approach has been the development of infrared drying units to rapidly dry less volatile solvents, such as water. Also, rapidly drying overcoats have been applied to protect slow drying inks while they dry normally. The ink need not dry instantaneously as long as the markings possess sufficient mechanical properties to prevent disfiguration.
The present invention is an approach enabling the use of water as the primary or sole ink vehicle or solvent for a "rapid drying" printing ink formation.
Various polymeric materials, including styrene/maleic anhydride copolymers have been employed as vehicle components in water-based printing inks. See "Inks and Coatings" by Pollock, Paper, Film and Foil Converter (September, 1975), page 50. Maleic anhydride copolymers are also known as dispersants for pigments. See, for example U.S. Pat. Nos. 2,980,653, 2,938,016, 2,913,437 and 2,857,365. Cross-linked maleic anhydride copolymers useable in the salt form as thickness in textile pastes are disclosed in U.S. Pat. Nos. 3,878,151, 3,165,485 and 3,283,033.
Maleimide copolymers, such as imides of isobutylene/maleic anhydride interpolymers are well documented in the patent literature. See U.S. Pat. No. 3,053,814 of the same inventor, including discussion of water solubility characteristics thereof. U.S. Pat. No. 3,053,814 states that the imide copolymer above containing a minimal imide content, is soluble in water containing ammonium hydroxide, but upon loss of ammonia the solution gels and the gels dry to water insoluble films. The possibility of using such a copolymer as a rapid drying component in an aqueous or water based printing ink environment is not suggested by the earlier Hedrick patent. Thus, there is still a need for an aqueous-based printing ink which can rapidly dry at room temperature, with acceleration by heating where desired.
It is also known to employ other salt forming polymers as gel-forming agents, for example, in floor waxes. Carboxyl-containing polymers can be dissolved in ammonia solution. As ammonia is lost after drying of the wax, the insoluble carboxylic acid resin forms. However, this type of system usually loses ammonia rather slowly (hence, the floor wax use) and because of this, heat would be necessary to accelerate the drying and insolubilization process. Vanderhoff in "Status of Ultraviolet Light-Cured Printing Inks", Journal of Radiation Curing, (October 1974), pp. 7-21, discusses the use of ammonium hydroxide-neutralized alkaline solutions of carboxyl-containing polymers to provide "tack" in water-based inks. As the ink dries through evaporation and/or absorption of the water, the viscosity of the ink increases to provide the "tack", or adherence to the substrate. Ammonia is slowly released to form the water-insoluble carboxylic polymer when the ink is dry. Vanderhoff discusses the "slow drying speed" of these water-based inks. As an ink organic solvent system version of a concept employing resin salt, see U.S. Pat. Nos. 3,412,053 and 2,720,461. In the latter patent, one embodiment involves an organic solvent based ink varnish containing a volatile amine salt of a water-insoluble and organic solvent-insoluble resin, said salt being soluble in the organic solvent based ink varnish. The free resin precipitates upon the printed sheet or is filtered onto the printed sheet through diffusion of the liquid vehicle into the sheet. An ink composed essentially of water as solvent is not considered, nor is there any consideration of a rapid gelling ink.